4th Gen Diamond or Pearl?

[Peanutz]

The only difference between the games is the Pokemon.
So, yeah, just decide which Pokemon you would want more. But, you can always trade.

I bought Diamond, and Dialga seems stronger anyway.

So, you could just buy Diamond or Pearl, and then maybe get Platinum when it comes out? Because that's what I'm doing.

Also, remember that Platinum is gonna have more features, and Giratina, Shaymin, and Regigigas are supposedly gonna have new forms.

 

[Glaceon_]

It comes out in the fall in Japan. Platinum will be out in the Spring for the US.

...and for UK, it will likely be out during the Summer, next year. If not, other side of October/November; up to, and beyond a year after Japanese release, because the European fan base is abused as such.